1. Field of the Invention
The present invention relates to an oscillator and, more particularly, to an assistant measuring circuit for the oscillator.
2. Description of the Prior Art
A voltage-controlled oscillator, which mainly provides reference signals with different phases but of an identical amplitude, plays an important role in a wireless communication system. The reference signals are used as carrier signals for transmitting data, and the qualities of the signals relate to the whole performance of the wireless communication system. Take a wireless communication system of an orthogonal frequency division multiplexing (OFDM) technique for example. The bit error rate of the communication system will increase if the amplitude differences and phase errors among the reference signals increase. For a phase array receiver, the poor quality signals affect the radiation pattern severely.
In addition to the fabrication of a good voltage-controlled oscillator, investigating how reference signals can be measured is also an indispensable issue. Only by measuring the amplitude differences and phase errors of the reference signals accurately, can a circuit designer or a manufacturer of a wireless communication equipment determine whether an output signal of certain oscillator is qualified or not.
There are not many measuring methods disclosed in the prior art for an output signal of an oscillator, especially for the oscillator whose output signals have more than four phase outputs. Recently, for measuring the amplitude differences and phase errors of multiple-phase signals, the main measuring method is to measure the output signal of each phase by using a high-speed sampling oscilloscope. The above-mentioned measuring method is manifested in “A 28-GHz monolithic integrated quadrature oscillator in SiGe Bipolar Technology” which was presented by Sabine Hackl, et al. in IEEE Journal of Solid-State Circuits, vol. 38, no. 1, pp. 135-137, January 2003.
However, restricted by the deficiency of an instrument's measuring frequency and the difficulty to read the small phase difference, it is not easy to determine precisely the amplitude differences and phase errors among the signals. Besides, with practical affairs such as setting up the instrument and calibrating a coaxial cable, it is also a challenge to measure multiple output signals simultaneously.
On the other hand, another measuring method is manifested in “A 45-GHz Quadrature Voltage Controlled Oscillator with a Reflection-Type IQ Modulator in 0.13 μm CMOS Technology” presented by H. Y. Chang, et al. in IEEE MTT-S Int. Microwave Symp. Dig., pp. 739-742, June 2006, and manifested in “A 32 GHz quadrature LC-VCO in 0.25 μm SiGe BiCMOS Technology” presented by W. L. Chan, et al. in 2005 International Solid-State Circuit Conference Digest, San Francisco, USA, pp. 538-539. This measuring method is to add a quadrature modulator operated as an image-rejection up-converter in the oscillating circuit, and to measure a side-band suppression of the quadrature modulator to calculate the amplitude differences and phase errors of the output signals.